[poppler] Bug in JBIG2Stream

[David Benjamin]

Hi,

So, I've been tracking down a bug in JBIG2Stream. It fails to render 
PDFs at [1] (second column) by misdetecting segments of incorrect length 
(and then eats up useful bytes at line 1445).  JBIG2Stream appears to 
bypass FilterStream::str and uses it's own curStr[2], so getPos() is 
wrong at the beginning.

Attached is a (trivial) patch to fix this. I'm not entirely sure if this 
is the "correct" thing to do here; the current setup feels pretty iffy 
anyway, but I get the feeling getPos() should not suddenly jump in the 
middle of a stream? I don't really know poppler's code, so I'm not sure 
what guarantees are Stream/FilterStream supposed to provide to users or 
if they're exported API in the first place.

(At least internally in JBIG2Stream the only non-error-reporting use of 
getPos is the segment-length-detection bit.)

[1] http://www.adobe.com/products/acrcapture/agentpack/index.html

[2] It seems to want to swap the stream out temporarily at reset()... 
I'm not sure why; I'm unfamiliar with JBIG2.


David Benjamin
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